Tube DIY Asylum

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I should have done better

Posted by sser2 on March 11, 2017 at 22:03:06:
searching the forum. I guess what you meant was the post by Lynn Olson (linked below).

Lynn's objection was that neutralization would only work under condition of perfect balance. Any imbalance (e.g. transition into Class B) will cause positive feedback exceeding negative feedback and causing bursts of oscillation.

This is a real problem indeed, but there are at least two ways to mitigate it. First, a differential stage can be forced into complete symmetry (pure Class A) by a CCS or a choke. Second, neutralizing capacitors can be made smaller than Cpg, which will cause partial rather than complete neutralization, but safeguard against excessive positive feedback.

Yet another source of problems could be imperfections in neutralizing capacitors introducing distortion at the critical signal juncture. However, since capacitances in question are relatively low, very high quality capacitors can be used. Vacuum caps come to mind.